A waveform of a pulse or a heartbeat reflects the heartbeat, a status of blood vessel, or a status of autonomic nerve. Information having high accuracy (such as an electrocardiogram) is not acquired from this waveform, which is only used as an index to diagnose a disease. However, a method to diagnose using this waveform is easy for a doctor. Accordingly, by using a fluctuation or a shape of the waveform, a study to evaluate change of the human body has been made.
As a method for detecting the waveform, a photoelectric pulse wave sensor is usually used for monitoring the number of heartbeat during a person's exercise. The photoelectric pulse wave sensor irradiates a light into a living body, and a reflected light or a transmitted light from the living body is measured. In this case, the light is absorbed by haemoglobin in the living body, and an intensity of the reflected light or the transmitted light changes in proportion to a blood amount. Accordingly, a signal waveform synchronized with the heartbeat is observed as a pulse wave. As to the pulse wave, in comparison with another index (For example, the electrocardiogram) for the living body, a pulse wave sensor is easily wearable and suitable to use in daily life.
From the wave pulse, a blood flow pulsed from the heart and passed to the peripherals via the artery is observed. Accordingly, if a blood vessel system is regular (normal), a pulsation interval of the pulse wave is assumed to be equal to a pulsation interval of the heart (heartbeat interval), and the pulse wave interval is often used in the same way as R-R interval (heartbeat interval) of the electrocardiogram. In this case, by converting pulse wave interval data to a frequency spectrum distribution, a power spectrum at a low frequency region (LF: 0.05˜0.15 Hz) and a high frequency region (HF: 0.15˜0.4 Hz) is calculated from the frequency spectrum distribution (a sequence of the pulse wave interval data). Thus, an index of the autonomic nerve (used for deciding a sleep condition) is acquired from a value of the power spectrum.
However, in case of analyzing a frequency at the pulse wave interval, when an arrhythmia is mixed onto the pulse wave, the arrhythmia is observed as a noise having a spike shape, and a noise element overlaps with a wide band of the frequency region. Accordingly, in case of analyzing the frequency, the pulse wave interval data corresponding to the arrhythmia need be eliminated.
As a method for detecting the arrhythmia in the pulse wave, a minimum and a maximum are extracted from the pulse wave, and a difference between the minimum and the maximum is calculated as am amplitude. The amplitude larger than a threshold is decided as a signal, and the amplitude not larger than the threshold is decided as a noise (arrhythmia). This method is disclosed in “JP-A 2007-125366 . . . Reference 1”.
Furthermore, as another method for detecting the arrhythmia, the arrhythmia itself is extracted from the pulse waveform observed, and used for diagnosis. This method is disclosed in “JP-A 2007-117591 . . . Reference 2” and “Japanese Patent No. 3635663 . . . Reference 3”.
In the Reference 2, when pulse wave intervals largely distribute in a predetermined period, the arrhythmia is included in the pulse wave data, i.e., the condition of a patient is decided as the arrhythmia. In the Reference 3, by analyzing a frequency of pulse wave intervals in a predetermined period, from characteristic of distribution (fluctuation) of the frequency, the arrhythmia is decided to be included, i.e., the condition of a patient is decided as the arrhythmia.
As mentioned-above, in the Reference 1, the arrhythmia is decided by comparing the amplitude (calculated from the pulse wave) with the threshold. However, the pulse wave signal has the amplitude with fluctuation. Furthermore, the amplitude is not always small in case of the arrhythmia. Accordingly, the arrhythmia cannot be correctly detected.
In the Reference 2, the arrhythmia is decided by characteristic of distribution of pulse wave intervals. In the Reference 3, the arrhythmia is decided by analysis result of frequency of pulse wave intervals. In both cases, the arrhythmia cannot be decided for each pulse of the pulse waveforms. Briefly, in the References 1˜3, the arrhythmia cannot be decided for each pulse by synthetically diagnosing the amplitude and the interval of pulse waveforms.